Sea anemones (scientific name: Actiniaria) are a major group of the Cnidaria, named after the anemone, a terrestrial flower. There are more than 1,000 species of sea anemone found throughout the world’s oceans at various depths, although the largest and most varied occur in coastal tropical waters. They can be almost any colour with most between 1.8 cm and 3 cm in diameter. They can be as small as 4 mm or as large as almost 200 cm across with a few tens of tentacles to hundreds.

.A sea anemone is a polyp attached at the bottom to the surface beneath it by an adhesive foot, called a pedal disk, with a column shaped body ending in an oral disk. The mouth is in the middle of the oral disk surrounded by tentacles armed with many nematocysts for defense and to capture prey.

A few species are free swimming and are not attached to the bottom; instead they have a gas chamber within the pedal disc, allowing them to float upside down in the water.

Sea anemones are carnivores that eat fish, mussels, zooplankton (like copepods, other small crustaceans, and tiny marine larvae), and worms. Sea anemones have few predators like some Sea slugs and some fish.

Some anemones have a symbiotic relationships with green algae. In exchange for providing the algae safety and access to sunlight, the anemone receives oxygen and sugar, the products of the algae's photosynthesis.

Sea anemones reproduce by lateral fission (in which an identical animal sprouts out of the anemone's side) and by sexual reproduction (in which anemones release eggs and sperm, producing free-swimming larvae).

Sea anemones and clownfishSea anemones live in a symbiotic relationship with the 29 species of clownfish. The clownfish are protected by a mucus layer that makes them immune to the anemone's sting. Clownfish live within the anemone’s tentacles, getting protection from predators, and the anemone eats the scraps from the clownfish’s meals. The clownfish also help keep the anemone's tentacles clean. Clownfish are yellow, orange, reddish or blackish, in colour often with white bars or patches. The largest can reach a length of 18 centimetres while some can be as small as 10 centimetres.

A group of clownfish normally live In an anemone. The group contains a breeding pair and some non-reproductive smaller males. When the female dies, the dominant male changes sex and becomes the female.